Conventionally, there are radar apparatuses which transmit a radio wave, and which receive a reflected wave that is produced by reflecting the transmission signal from an object, thereby identifying the position of the object. In radar apparatuses of this kind, a pulse signal which has been converted to a radio frequency is often used as the transmission signal. The signal power of a pulse signal attenuates during a period from transmission to reception after reflection from an object.
When the reflected wave is a wave reflected from an object at a closer distance, therefore, the power of the reception signal is higher, and, when the reflected wave is that reflected from an object at a remoter distance, the power is lower. Consequently, a radar apparatus requires a circuit for amplifying a reflected wave from an object at a remote distance. Conventionally, an STC (Sensitive-Time-Control) circuit has been used as an amplification circuit of a radar apparatus.
FIG. 17 is a block diagram showing the configuration of a radar apparatus of a conventional example. In the radar apparatus, a transmission signal which is a pulse signal generated in a transmitter 1001 is transmitted into a space from an antenna 1003 via a transmission/reception switching device 1002. A reception signal which is received from the space by the antenna 1003 is input to a receiver 1004 via the transmission/reception switching device 1002. In the receiver 1004, the reception signal is amplified in amplifiers 1005, 1006, and subjected to a demodulation process, thereby obtaining a demodulated signal. The demodulated reception signal is sent to an indicator 1007 such as a monitor device to be displayed thereon (see Patent Document 1).
FIGS. 18(a) to 18(c) are waveform charts illustrating the operation of the radar apparatus of the conventional example. FIG. 18(a) shows a reception signal in the radar apparatus. In the figure, the ordinate indicates the reception power, and the abscissa indicates the distance (time). In FIG. 18(a), the manner is shown in which the pulse signal transmitted at time 0 is received while attenuating with elapse of time.
In FIG. 18(a), e1, e2, and e3 indicate a strong reflected wave from a reflection object. Although the reflection coefficient of a radio wave depends on the size and shape of a reflection object, the signal power of a reception signal in a radar apparatus is largely affected by the distance of the reflection object.
FIG. 18(b) shows the amplification factor of an amplifier of the receiver of the radar apparatus. In the figure, the ordinate indicates the amplification factor, and the abscissa indicates the distance (time). The amplification factor in amplification of the reception signal is controlled so as to be increased with the elapsed time from the transmission of the pulse signal. When the reflected wave is received from a near object, therefore, the amplification factor is reduced, and, when the reflected wave is received from a remote object, the amplification factor is increased.
FIG. 18(c) shows the signal power after the amplification in the radar apparatus. In the figure, the ordinate indicates the signal power, and the abscissa indicates the distance (time). In this way, the difference in reception power between the reflected wave from a close distance and that from a remote distance can be made small.
When the amplification factor of the amplifier is adjusted in accordance with the distance (time) by using the above-described method, the dynamic range of the reception signal can be reduced, and the quantization bit number of an AD converter which is continuous to the rear stage of the receiver can be reduced.
Patent Document 2 discloses a configuration where, in place of the method of adjusting the amplification factor of an amplifier, the attenuation factor of an attenuator (STS attenuator) connected to the front or rear stage of the amplifier is changed. In the configuration, contrary to the method of adjusting the amplification factor of an amplifier shown in FIG. 18(b), a control in which the attenuation factor of the attenuator is reduced with elapse of time is performed. According to the configuration, it is possible to obtain a signal similar to the reception signal shown in FIG. 18(c).